The present invention relates to the stabilization of polyolefin nonwoven webs. More particularly, the present invention relates to the stabilization of polyolefin nonwoven webs against the deleterious effects of actinic radiation.
Nonwoven webs are employed in a wide variety of applications, with the largest category being disposable absorbent products. However, nonwoven webs also are found in products which are intended for use in an external environment, i.e. outdoors. Examples of such products include agricultural row covers, tent fabrics, protective automobile covers, and the like. Many of these products are exposed to sunlight for long periods of time. Consequently, such products often must be stable against the deleterious effects of actinic radiation, especially ultraviolet radiation.
It has been known for many years that nonwoven webs prepared from thermoplastic polymers can be given some degree of stability by incorporating a stabilizer into the polymer. Such stabilizers typically are distributed through out the bulk of the fibers. While such stabilizers have a degree of effectiveness, relatively high concentrations often must be used in order to get a sufficiently high degree of stabilization.
A novel way to avoid the use of high stabilizer concentrations is described in U.S. Pat. No. 4,923,914 to Nohr et al., which patent is incorporated herein by reference. The patent describes a surface-segregatable, melt-extrudable thermoplastic composition which comprises at least one thermoplastic polymer and at least one defined additive. The additive can be a polysiloxane having a benzotriazolyl substituent or a tetraalkylpiperidyl substituent. Benzotriazoles are known absorbers of ultraviolet radiation, whereas tetraalkylpiperidines are known to function by deactivating excited oxygen molecules or terminating free radicals.
Upon being melt-extruded, the compositions of U.S. Pat. No. 4,923,914 result in fibers having a differential, increasing concentration of the additive from the centers to the surfaces thereof, such that the concentration of additive toward the surface of each fiber is greater than the average concentration of additive in the more central region of the fiber and imparts to the surface of the fiber at least one desired characteristic which otherwise would not be present. The additive is miscible with the polymer at melt extrusion temperatures, under which conditions the additive and the polymer form a metastable solution. As the temperature of the newly formed fiber drops below melt extrusion temperatures, the additive becomes significantly less compatible with the polymer. Concurrent with this marked change in compatibility, the polymer begins to solidify. Both factors contribute to the rapid migration or segregation of the additive toward the surface which takes place in a controllable manner.
The patent refers to the use of different molecular weight additives in order to achieve a complimentary or even synergistic effect. For example, a first additive could be a polysiloxane having a benzotriazolyl substituent and a second additive could be a polysiloxane having a tetraalkylpiperidyl substituent. The molecular weight of the first additive would be chosen to result in the migration of the additive primarily to the interfacial surfaces and effective surfaces of the fibers. The molecular weight of the second additive, however, would be chosen to result in the migration of the additive primarily to the subsurface. According to the patent, radiation which is not absorbed by the first additive would be nullified by the second additive.
Actinic radiation, however, often causes significant reductions in the tensile properties of fibers because of the degradation of polymer throughout the fiber. While the method of stabilizing fibers described in U.S. Pat. No. 4,923,914 as summarized above certainly will delay losses of tensile properties, free radicals which migrate deeper than the subsurface of a fiber in time will adversely affect the tensile properties of the fibers.